In a LTE system, a UE is always connected to a cellular network, and transmits or receives a Device-to-Device (D2D) signal or channel.
D2D communication under a LTE cell occupies uplink resources, i.e., Frequency Division Duplexing (FDD) uplink carriers or Time Division Duplexing (TDD) uplink subframes. Communication channels or signals of the cellular network may include a Physical Uplink Shared Channel (PUSCH), a Physical Uplink Control Channel (PUCCH), a Physical Random Access Channel (PRACH) and a Sounding Reference Signal (SRS). D2D channels or signals may include a Synchronization Signal (SS), a Discovery Signal (DS), a Control Channel (CCH) and a Data Channel (DCH). A D2D SS may include one or more of a Primary D2D SS (PD2DSS), a Secondary D2D SS (SD2DSS) and a Physical D2D Synchronization Channel (PD2DSCH). A D2D DS may include a D2D DS sequence and a D2D DS message.
A D2D UE may have different service requirements, for example, a service requirement for Public Safety (PS) or a service requirement for Non-public Safety (N-PS). Different service requirements have different importance.
It is on the same uplink subframe that a UE is to transmit or receive a D2D channel or signal and transmit a cellular channel or signal. The UE is unable to transmit both a D2D channel or signal and a cellular channel or signal at the same time in a cell through a Frequency Division Multiplexing (FDM) mode. However, on a D2D subframe, the D2D channel or signal needs to be transmitted or received, and HARQ-ACK generated by a Physical Downlink Shared Channel (PDSCH) on a downlink subframe corresponding to the uplink subframe also needs to be transmitted.